Electronic devices and electro-mechanical devices are devices which can include both electrical and mechanical parts. Sometimes, because of the nature of these parts, the way they interact with one another, and the environment within which they are used, the parts can be adversely affected. Exemplary electronic and electro-mechanical devices in the form of force-sensing transducers are described in U.S. Pat. Nos. 5,367,217, 5,339,698, and 5,331,242, the disclosures of which are incorporated by reference. Exemplary accelerometers are described in U.S. Pat. Nos. 5,594,170, 5,501,103, 5,379,639, 5,377,545, 5,456,111, 5,456,110, and 5,005,413, which are incorporated by reference herein. Other types of electronic and electro-mechanical devices can be used as rate sensors. Exemplary rate sensors are described in U.S. Pat. Nos. 5,717,140, 5,376,217, 5,696,323, 5,691,472, and 5,668,329, which are incorporated by reference herein. Yet other electronic or electro-mechanical devices can be used as acceleration and rate sensors. Exemplary sensors are described in U.S. Pat. Nos. 5,627,314, 5,557,046, 5,341,682, 5,331,853, 5,331,854, and 5,319,976, the disclosures of which are incorporated by reference herein.
One type of electronic or electro-mechanical device is an accelerometer. Accelerometers can have mechanical parts which experience acceleration forces, and electrical parts to help set up conditions conducive to sensing such forces, and to assist in sensing such forces. Yet, these electrical and mechanical parts can sometimes experience problems which adversely impact their performance.
For example, micromachined silicon accelerometers can exhibit undesirable resonator frequency drift at temperatures below 200.degree. C. and even at room temperature. This drift is believed to be due to "creep" or plastic deformation in a current-carrying, evaporated chromium/gold or chromium/molybdenum/goldmetallization layer following 400.degree. C. wafer bonding in air. This drift can degrade the performance of a device and can limit its accuracy and its marketability.
The frequency drift can be directly attributed to structural changes which take place in the top, thicker gold layer (typically around 0.2-0.7 micron in thickness). The plastic deformations and creep in the gold layer are due to a low Young's modulus, low hardness and low tensile strength of gold. A much thinner chromium layer (around 100 Angstrom) is used to improve the adhesion of the gold or molybdenum to an underlying thermal silicon dioxide (SiO.sub.2) and does not appreciably contribute to the overall resistance or current-carrying capacity. In addition to the above-described creep, gold and chromium, or gold and molybdenum in thin-film form show significant undesirable interdiffusion among themselves in the above temperature range. This can result in an undesirable increased resistivity and visual splotchiness in an otherwise uniform mirror-like appearance of the layers.
Accordingly, there is a need for a more stable metallization, and one which can mitigate some of the problems experienced in electronic or electro-mechanical devices having mechanical and electrical parts. Particularly, there is a need within the context of accelerometers for stable metallization which would mitigate resonator drift frequency, and which would have reduced or no interdiffusion when in contact with chromium or other adhesion-promoting layers. Further, there is a need for materials which exhibit stable resistivity and visual appearance. In addition, an improved metallization scheme should desirably exhibit relatively low electrical sheet resistance, e.g. about 0.25 ohm/square at a total thickness of less than one micron, and good bondability to gold wire. Stress in the metallization should also be desirably as low as possible.